The present invention relates broadly to a strain gage apparatus, and in particular to a three-wire static strain gage apparatus.
In the prior art, it has been well understood that the electrical resistance of electrically conductive metals varies in accordance with the function of the strain to which they are subjected. This phenomenon is termed strain-sensitivity and has been utilized in the so-called metal foil and wire-wound types of strain gages. These strain gages generally employ a length or a plurality of series-connected parallel lengths of fine wire, which is known as a grid or filament, that is bonded by a suitable adhesive to the member to be stressed wherein the strain on the member is imparted to the wire. The resultant strain causes a variation in the electrical resistance of the strain sensitive wire that is usually measured by a Wheatstone bridge in which the strained wire comprises one leg of the bridge. The bridge recording or indicating device, such as a galvanometer, may be conveniently calibrated to give a reading of strain in inches per inch which may be computed to provide stress in pounds per square inch.
Whenever a strain gage is bonded to a test member for the purpose of measuring the strain of the member due to varying load conditions, the overall strain reading may be adversely affected by a condition that is commonly known as the apparent strain. The apparent strain may be defined as a strain indication that is generated in response to a thermal expansion of the material engaged, the temperature resistance coefficient of the strain gage wire, and changes in strain gage lead wire resistance due to temperature variations. It may be seen that the apparent strain is undesirable since the strain reading will not represent a true conversion of stress and strain forces which are placed on the test member but will include expansion effects that are caused by variations in temperature.
One conventional prior art method of temperature compensation is to employ a strain sensing gage which is constructed made of specially selected wire materials which can be used on test members within a predetermined temperature range for which the gage materials have been selected. This method has been successfully employed for a limited temperature range and generally required controlled test conditions. Another technique for providing temperature compensation involves the employment of a dummy gage which is placed in close proximity to the strain sensing gage. The dummy gage is generally inserted in an adjacent leg of the Wheatstone bridge circuit so that a cancellation occurs when the resistance values and the temperature changes are at all times identical. However, because of temperature differences between the sensing strain gage and the dummy gage due to convective air currents for example, the resistance of the two gages are not equal and compensation for such transit temperatures cannot be readily obtained.